1. Field of the Invention
The present invention relates to an apparatus and method for inspecting a pixel driving thin film transistor (hereinafter referred to as TFT) which is used for, for example, a liquid crystal display (hereinafter referred to as LCD) of an active matrix type.
2. Description of the Related Art
In recent years, in the LCD of the active matrix type which is particularly in the spotlight, a TFT is usually utilized for driving pixels thereof.
The TFT is manufactured by the use of a high-level thin film formation technique. However, it is difficult to eliminate manufacturing defects completely, and attention has been given to improvements in an apparatus and a method for inspecting the TFT.
Particularly, it is very difficult to connect terminals of the inspection apparatus directly to all of the drain electrodes or drive electrodes of the TFT, and it is regarded as important how this problem is solved.
Hence, as apparatuses for inspecting the pixel driving TFT to be used for a substrate of the LCD, following apparatuses have been used.
(a) an apparatus for measuring characteristics of an inspection TFT formed at a portion other than a display portion on the substrate (a curve tracer for use in a general transistor); PA1 (b) an apparatus for detecting a current with a tunnel effect by bringing a detection terminal close to a transparent electrode connected to a drain electrode of the TFT; and PA1 (c) an apparatus for measuring characteristics of the TFT utilizing electro-optic effect.
According to these apparatuses and methods utilizing the same, it is not necessary to connect a terminal of the inspection apparatus directly to the drain electrode of the TFT. For this reason, however, these apparatuses have the following problems. The apparatus (a) cannot inspect the TFT of the display portion. The apparatus (b) is poor in reproducibility and accuracy. The apparatuses (b) and (c) become expensive and large-scaled, and take a very long measuring time.
An invention made to solve the above problems has been applied for a patent (Japanese Unexamined Patent Publication No. 5-90373).
The invention relates to an inspection apparatus which charges a capacitative element connected to the drain electrode of the TFT and then, measures the stored charge as voltage or as current.
However, the inspection apparatus is based on the assumption that the capacitative element is connected to the drain electrode, and has the problem that the apparatus is insufficient to the TFT in which the capacitative element is not connected to the drain electrode with respect to accuracy and reproducibility.